"Our results have demonstrated that as the sea ice goes, so goes the polar bear," said Steven Amstrup, a U.S. Geological Survey (USGS) wildlife research biologist in Anchorage, Alaska, and leader of the polar bear studies.

USGS conducted the studies to help the U.S. Fish and Wildlife Service determine whether polar bears warrant protection under the U.S. Endangered Species Act. That decision is due in January 2008.

Kassie Siegel is a climate change activist with the Center for Biological Diversity in Joshua Tree, California. The new studies, she said, represent a watershed moment in the climate crisis.

"If we don't change the path that we're on now, then it will be too late," she said. "Polar bears will become extinct."

Polar Bears and Sea IceScientists estimate that 20,000 to 25,000 polar bears live throughout the Northern Hemisphere in areas that are covered by sea ice for extensive periods.

Polar bears eat mostly seals and other fatty marine mammals that they hunt from the ice.In some places—like the southern end of polar bear range in Canada's Hudson Bay—the sea ice melts in the summer. The bears then come onto land, where they have insufficient food until the sea ice refreezes in the fall.

Those bears, USGS's Amstrup said, may be the first to die off. As the sea ice melts sooner each summer, the bears will be forced to come ashore earlier and face food shortages before they have stored enough fat to last through the season.

"There's a limit to how long they can fast," Amstrup said.

In other regions polar bears live on the sea ice year-round.

These bears prefer to hunt from the ice that hangs over shallow continental shelf waters, which contain more prey than deeper waters offshore.

But as the ice retreats farther from the shoreline, the bears may have less success with their hunts (related: "Arctic Ice at All-Time Low" [August 20, 2007]).

In addition, the rapid movement of sea ice in these regions may leave some bears stranded in food-poor areas, or it might require long and exhausting swims from food-rich areas back to ice, Amstrup noted.

By mid-century polar bears will likely be eliminated from the southern end of their range and in the Polar Basin, where sea ice is moving farther from land.

These two areas are currently home to approximately two-thirds of the world's polar bear population, Amstrup said.

Polar bears are only predicted to hang on in the Canadian Arctic islands and some regions along the northern coast of Greenland where sea ice is expected to persist even in summer (see map).

Survival HopesThe USGS findings are based on the best available models of future climate, Amstrup said.

Warming temperatures, however, are melting the sea ice quicker than even the best climate models projected, according to the National Snow and Ice Data Center in Boulder, Colorado.

As of September 3, 2007, sea-ice extent had fallen to 1.70 million square miles (4.42 million square kilometers)—beating the previous record low of 2.05 million square miles (5.32 million square kilometers) set on September 21, 2005.